Enhanced Turbocompressor Startup

ABSTRACT

A control method and apparatus for startup of turbocompressors to avoid overpowering a driver of the turbocompressor. In a first embodiment, the control system monitors input signals from transmitters of various control inputs. When the input signals exceed threshold values, the control system begins to close the antisurge valve. In a second embodiment, the antisurge valve begins to close after a predetermined time measured from the time startup is initiated. In both embodiments, the antisurge valve continues to ramp closed until the compressor has reached its operating zone, or until the compressor&#39;s operating point reaches a surge control line, at which point the antisurge valve is manipulated to keep the compressor from surging.

CROSS REFERENCE TO RELATED APPLICATIONS

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a control scheme. Moreparticularly the present invention relates to a method and apparatus forreducing a shaft power required when starting up a turbocompressor bymanipulating the compressor's antisurge recycle valve.

2. Background Art

As shown in FIG. 1, a turbocompressor 100, whether axial or centrifugal,is driven by a driver such as a variable speed electric motor 110. Arecycle valve 120, used for antisurge protection, is piped in parallelwith the compressor 100. An inlet throttling valve 130 may be used forcompressor capacity or performance control.

As all those of ordinary skill in this art know, surge is an unstableoperating condition of a turbocompressor encountered at generally lowflow rates. The surge region is shown in FIG. 2 to the left of the surgelimit curve 210. In FIG. 2, H_(p) is the polytropic head and Q is thevolumetric flow rate, both associated with the turbocompressor.

For the purposes of this document, including the claims, thecompressor's minimum operating speed is hereby defined as the minimumrotational speed, greater than idle speed, at which the compressor maybe operated continuously. The minimum operating speed is defined by thecompressor manufacturer. It is generally depicted as the lowestperformance curve in a compressor performance map such as shown in FIGS.2 and 3. Lower speeds, greater than idle speed, are experienced onstartup and shutdown, but the compressor is not operated continuously atthese speeds. For turbocompressors operated at a constant speed, such asthose driven by constant speed electric motors, the minimum operatingspeed is simply the constant operating rotational speed.

As those of ordinary skill know, the accepted startup procedure for aturbocompressor is to increase the rotational speed of the compressorwith the antisurge valve 120 wide open until the compressor reaches thecompressor's minimum operating speed (if the compressor is operated atvariable speed) or the compressor's operating speed (if the compressoris driven by a constant speed driver). At this point in the startupprocedure, the antisurge valve 120 is ramped closed and the compressor's100 automatic performance control takes control of the compressor'srotational speed, inlet throttling valve 130, or variable guide vanes tocontrol the compressor's 100 capacity.

As is recognized by all those of ordinary skill, this startup procedureprovides the most safety for the compressor because surge will beavoided, as depicted in FIG. 3. The compressor's 100 operating pointtrajectory 320 is shown as a dot-dashed line. Curves of constantcompressor rotational speed 310 a-310 e are shown as solid lines. Thecurve 310 a represents the minimum operating rotational speed, while thecurve 310 e represents the maximum operating rotational speed. Becausethe recycle valve 120 is maintained in its fully open position untilminimum rotational speed has been achieved, the compressor operatingpoint trajectory 320 tends to give wide berth to the surge limit curve210 in the region below the minimum rotational speed curve 310 a.

Additional impetuses for startup with the antisurge valve 120 fully openare that the surge limit curve 210 is usually unknown for rotationalspeeds less than the minimum operating speed, and that pressure and flowsensor signals of reasonable magnitude must be achieved before a validcompressor operating point may be determined. The compressor's operatingpoint must be calculated to compare its location to the surge limit line210, or surge control line 220 to avoid having the compressor'soperating point cross the surge limit line 210. Antisurge controlalgorithms are described in the Compressor Controls Series 5 AntisurgeControl Application Manual, Publication UM5411 rev. 2.8.0 December 2007,herein incorporated in its entirety by reference.

Due to the large flow through the compressor 100 during startup usingthe above standard procedure, the shaft power required to drive thecompressor 100 is large. This results in slower startup and, possibly,tripping of the driver due to power overload.

A gas turbine driver may experience high exhaust gas temperatures duringthe startup of a turbocompressor. An electric motor driver may trip onthermal overload due to a current being too high for too long aduration.

There is, therefore, a need for an improved control strategy for thestartup of turbocompressors to reduce the loading of the compressorwhile maintaining the compressor flow out of the unstable, surge region.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and apparatusfor safely starting a turbocompressor while minimizing an overall energyrequired to accomplish the startup.

Compressors having gas turbine drivers and variable frequency driveelectric motors tend to have long startup times—on the order of severalminutes. For this class of compressors, a first embodiment of thisinvention prescribes that the compressor's antisurge valve be maintainedat its fully open position until predetermined signal strengths arerealized from the compressor's suction and discharge pressure sensors,and the flow sensor. At this point, the antisurge valve is ramped closedat a predetermined rate under control of the antisurge control system tokeep the compressor's operating point from crossing the surge controlcurve. Startup continues independently of the antisurge controller'soperation.

A second class of compressors comprises constant-speed electric motordriven compressors. The startup times for this class of compressors tendto be on the order of less than a minute. In this case, the controlsystem starts the antisurge valve in a fully open position, and beginsto ramp the antisurge valve closed at a predetermined rate after apredetermined time has elapsed after the initiation of the startup ofthe compressor. Because of the rapid startup, the pressure and flowsensor signals become viable very quickly, so antisurge control may becarried out before the compressor's operating point reaches the surgecontrol curve.

The novel features which are believed to be characteristic of thisinvention, both as to its organization and method of operation togetherwith further objectives and advantages thereto, will be betterunderstood from the following description considered in connection withthe accompanying drawings in which a presently preferred embodiment ofthe invention is illustrated by way of example. It is to be expresslyunderstood however, that the drawings are for the purpose ofillustration and description only and not intended as a definition ofthe limits of the invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic of a compressor, driver, and antisurge recyclevalve;

FIG. 2 is a first representative compressor performance map;

FIG. 3 is a second representative compressor performance map showing afirst compressor operating point's startup trajectory;

FIG. 4 is a is a third representative compressor performance map showinglines of constant shaft power;

FIG. 5 is a fourth representative compressor performance map showing asecond compressor operating point's startup trajectory;

FIG. 6 a is a schematic of a variable speed motor driven compressorsystem;

FIG. 6 b is a schematic of a constant speed motor driven compressorsystem;

FIG. 7 is a schematic of a turbine driven compressor system;

FIG. 8 is a flow diagram of a first embodiment of the present invention;

FIG. 9 is a flow diagram of a second embodiment of the presentinvention;

FIG. 10 is a flow diagram of a third embodiment of the presentinvention; and

FIG. 11 is a detail flow diagram of a startup initiation process

DETAILED DESCRIPTION OF THE INVENTION

A typical compressor performance map in H_(p)-Q coordinates is shown inFIG. 4. Here, H_(p) is polytropic head and Q is volumetric flowrate—usually in the suction. The map of FIG. 4 comprises solid-linecurves of constant rotational speed 310 a-310 e and dashed-line curvesof constant shaft power 410 a-410 e. As is clear from the relationshipbetween the curves of constant rotational speed 310 a-310 e and thecurves of constant shaft power 410 a-410 e, at a given rotational speed,the required shaft power decreases as the operating point moves towardthe surge limit 210. To avoid overpowering the compressor driver 110,710 (see FIG. 7) an operating point trajectory 520, shown in FIG. 5,running as near the surge limit 210 as possible, should be used. Theshort-dashed curve 510 represents a surge control line—a line set apredetermined distance from the surge limit line 210 toward the stableoperating region, thus providing a safety margin for the antisurgecontrol system.

As those of ordinary skill in the art of compressor control know, limitcontrol is applied to the compressor 100 to maintain the operating pointat or to the right of the surge control line 510. To effect thiscontrol, an antisurge or recycle valve 120, as shown in FIGS. 1, 6 a, 6b, and 7, is manipulated to maintain an adequate flow rate through thecompressor 100. The manipulation of the antisurge valve 120 is carriedout via an automatic control algorithm in the antisurge controller, A/SPID 610, of FIGS. 6 a, 6 b, and 7. Typical inputs to the antisurgecontroller 610 are shown in FIGS. 6 a, 6 b, and 7 and comprise adifferential pressure signal from a flow transmitter, FT 620, a suctionpressure signal from a suction pressure transmitter, PT1 630, adischarge pressure signal from a discharge pressure transmitter, PT2635, and a rotational speed signal from speed pickup, SE 640 when thedriver is variable speed as in FIGS. 6 a, and 7. Often, in applicationsusing a constant speed driver, such as a constant speed electric motor640, as shown in FIG. 6, no speed pickup SE 640 in included.

To emulate the operating point trajectory 520 depicted in FIG. 5, theantisurge valve 120 is initially fully open, but is ramped closed by thecontrol system as soon as safe operation may be assured. One embodimentof the instant invention is depicted in the flow diagram of FIG. 8. Thisembodiment is particularly useful when the startup process is “slow,”taking on the order of several minutes from its initiation. Asmentioned, the antisurge valve 120 is set initially at its full openposition as shown in block 800. The full open position may vary betweenvalve types. Generally, full open in the context of this invention isthe greatest opening the antisurge valve 120 will realize in its duty inthe specific application. The present invention does not depend on thepercent opening value at which the antisurge valve 120 is considered inits full open position.

When the antisurge valve 120 is assured fully open, startup can beinitiated as shown in block 805. At startup, the rotational speed of thecompressor 100 is increased according to the guidelines and restrictionsof the compressor 100 and driver 110, 710 manufacturers and the needs ofthe equipment owner. In particular, critical speeds, if any, areconsidered and the startup schedule takes these speeds intoconsideration. Speed increase is depicted in block 810, and is effected,as shown in FIG. 11, by increasing a compressor speed set point used bya Variable Frequency Drive (VFD) controller 650 (FIG. 6 a) or arotational speed controller 720 (FIG. 7).

As the compressor speed increases, the control system 610 repeatedlychecks the signals received from the flow transmitter 620, suctionpressure transmitter 630, and discharge pressure transmitter 635. Thesignal values are compared to threshold values, Δp_(o,min), p_(s,min),and p_(d,min), respectively in comparator blocks 815, 820, 825. If thesignal magnitude of one or more of the input signals, Δp_(o), p_(s), andp_(d), is not at least as great as its respective threshold value, therotational speed of the compressor 100 continues to be ramped up asindicated in block 810.

Once all three signals, Δp_(o,min), p_(s,min), and p_(d,min), exceedtheir threshold values Δp_(o,min), p_(s,min), and p_(d,min), twooperations are carried out essentially simultaneously and repeatedly.Each of these operations emanates from and returns to the branch block830. In one of these operations, the antisurge controller 610 comparesthe compressor's operating point to the surge control line 510 todetermine how the antisurge valve 120 must be manipulated for antisurgeprotection. If the compressor's operating point is to the right of thesurge control line 510 as determined in the comparator block 835, theantisurge valve 120 is ramped toward its closed position according to apredetermined schedule as shown in block 850. If the operating point ison or to the left of the surge control line 510, the antisurgecontroller 610 manipulates the antisurge valve's 120 position to keepthe compressor 100 safe from surge as shown in block 845.

The other essentially simultaneous operation involves continuing toincrease the compressor's rotational speed according to block 855 untilthe minimum operating speed, N_(min), or some predetermined value ofspeed is reached. Continuing to increase the compressor's rotationalspeed is effected as explained with regard to block 810: the rotationalspeed set point used by the VFD controller 650 or the speed controller720 is increased with time. Those of ordinary skill in this art areintimate with this aspect of startup control. When the comparator block840 determines the compressor 100 has reached its minimum operatingspeed, the control system is shifted from its startup mode to its RUNmode, as shown in block 860. At that point, the capacity or performancecontrol system takes over varying the compressor speed according to theneeds of the process. Note that the minimum operating speed, N_(min), incomparator block 840 may be the compressor's operating speed if thecompressor 120 is to be operated at a constant speed.

An additional embodiment is shown in FIG. 9. This embodiment isparticularly useful for compressors 120 that may be started rapidly—inless than a minute, for instance. The antisurge valve 120 is setinitially at its full open position as shown in block 800. In block 910,a timer is reset to zero.

When the antisurge valve 120 is assured fully open and the timer hasbeen initialized, startup can be initiated as shown in block 805. Atstartup, the rotational speed of the compressor 100 is ramped upaccording to the guidelines and restrictions of the compressor 100 anddriver 110, 710 manufacturers and the needs of the equipment owner.Speed rampup is carried out by increasing the VFD controller's 650 orrotational speed controller's 720 set point, and is depicted in block810.

In this embodiment of the invention, the antisurge valve is rampedtoward a closed position after a predetermined time elapses. Incomparator block 920, the time as reported by the timer is compared tothe time threshold, t_(PD). If the time does not exceed the thresholdtime, the speed continues to increase, but no change to the position ofthe antisurge valve 120 is made. When the threshold time, t_(PD), haselapsed, two operations are carried out essentially simultaneously andrepeatedly. Each of these operations emanates from and returns to thebranch block 830. In one of these operations, the antisurge controller610 compares the compressor's operating point to the surge control line510 to determine how the antisurge valve 120 must be manipulated forantisurge protection. If the compressor's operating point is to theright of the surge control line 510 as determined in the comparatorblock 835, the antisurge valve 120 is ramped toward its closed positionaccording to a predetermined ramp rate as shown in block 850. If theoperating point is on or to the left of the surge control line 510, theantisurge controller 610 manipulates the antisurge valve's 120 positionto keep the compressor 100 safe from surge as shown in block 845.

The other essentially simultaneous operation involves continuing toincrease the compressor's rotational speed according to block 855 untilthe minimum operating speed, N_(min), or some predetermined value ofspeed is reached. When the comparator block 840 determines thecompressor 120 has reached its minimum operating speed, the controlsystem is shifted from its startup mode to its RUN mode, as shown inblock 860. At that point, the capacity or performance control systemtakes over varying the compressor speed according to the needs of theprocess. Note that the minimum operating speed, N_(min), in comparatorblock 840 may be the compressor's operating speed if the compressor 120is to be operated at a constant speed.

In FIG. 10, a third embodiment is illustrated, differing from theembodiment of FIG. 9 in that the driver of FIG. 10 is a constant speeddriver, such as a constant speed electric motor 640 (FIG. 6 b). In thisembodiment, the process of accelerating the driver up to its operatingspeed, N_(op), does not incorporate a decision to continue acceleratingthe driver inasmuch as the driver will continue to accelerate until itsoperating speed, N_(op), is reached or it is tripped. Therefore, block1055 indicates only that the rotational speed continues to rise. Block1040 is intended only to indicate the compressor rotational speed willincrease until the operating speed, N_(op), is reached, and not that adecision is being made in this comparator block. Ultimately, when thecompressor has reached its operating speed, N_(op), the control systemreverts to a RUN mode 860 wherein performance or capacity control iscarried out to satisfy process constraints. Note that, in this caseespecially, the predetermined time lapse, t_(PD), in comparator block920 may be zero so the antisurge valve 120 begins to close immediatelyas startup begins.

The last two embodiments differ from the prior art in that, in theinstant invention, time is used to determine when the antisurge valve120 is ramped toward its closed position, rather than rotational speed.

The flow diagrams in FIGS. 8, 9 and 10 may be considered contents of alogic unit within a compressor control system, such as the antisurgecontroller 610 depicted in FIGS. 6 a, 6 b, and 7.

More detail of the startup initiation block 810 is shown in FIG. 11. Acheck to ascertain the antisurge valve 120 is fully open is firstcarried out in query block 1110. If the antisurge valve 120 is not fullyopen, the flow moves to a valve open function 1120. Once the antisurgevalve 120 is fully open, the turbocompressor rotational speed isincreased from an initial, zero value as shown in block 1130.

The above embodiments are the preferred embodiments, but this inventionis not limited thereto. It is, therefore, apparent that manymodifications and variations of the present invention are possible inlight of the above teachings. It is, therefore, to be understood thatwithin the scope of the appended claims, the invention may be practicedotherwise than as specifically described.

1. A method of minimizing an energy required to start a turbocompressorhaving an antisurge valve and a control system, the method comprising:(a) opening the antisurge valve fully; (b) initiating a startup of theturbocompressor; (c) increasing a rotational speed of theturbocompressor from a zero rotational speed; and (d) actuating theantisurge valve toward its closed position before said turbocompressorreaches a minimum operating rotational speed.
 2. The method of claim 1wherein an initiation of a closing of the antisurge valve comprises: (a)sensing at least one signal by the control system; (b) comparing amagnitude of the at least one signal with a minimum threshold value; and(c) initiating the closing of the antisurge valve when the magnitude ofthe at least one signal exceeds the minimum threshold value.
 3. Themethod of claim 1 wherein an initiation of a closing of the antisurgevalve comprises: (a) initializing a timer to an initial value at a timeof initiating the startup of the turbocompressor; (b) measuring timeelapsed from the initial value; (c) comparing the measured time with athreshold value; and (d) initiating the closing of the antisurge valvewhen the elapsed time exceeds the threshold value.
 4. The method ofclaim 3 wherein the minimum operating rotational speed comprises aconstant operating rotational speed of the turbocompressor, the methodadditionally comprising: (a) permitting a driver of the turbocompressorto reach the constant operating rotational speed; and (b) operating theturbocompressor at the constant operating speed.
 5. The method of claim1 wherein increasing a rotational speed of the turbocompressor from azero rotational speed comprises: (a) increasing a turbocompressorrotational speed set point within the control system; and (b)controlling the turbocompressor rotational speed based on saidturbocompressor rotational speed set point.
 6. The method of claim 3wherein the threshold value is equal to the initial value.
 7. The methodof claim 1 wherein an initiation of a closing of the antisurge valvecomprises: (a) sensing a plurality of signals input to the controlsystem; (b) comparing magnitudes of each of said plurality of signalswith a respective minimum threshold value; and (c) initiating theclosing of the antisurge valve when the magnitudes of the plurality ofsignals exceed the respective minimum threshold values.
 8. The method ofclaim 1 wherein actuating the antisurge valve towards its closedposition before said turbocompressor reaches a minimum operatingrotational speed comprises: (a) detecting a signal indicating aturbocompressor operating point; (b) comparing said signal to a valuerepresenting a surge control line; and (c) manipulating the antisurgevalve to keep the turbocompressor operating point from residing nearer asurge region than the surge control line.
 9. An apparatus for minimizingan energy required to start a turbocompressor, the apparatus comprising:(a) an antisurge valve; (b) a control system; (c) a first control systemfunction to generate a first signal to open the antisurge valve fully;(d) a second control system function to initiate a startup of theturbocompressor and increase a rotational speed set point of theturbocompressor from a zero rotational speed set point; (e) a thirdcontrol system function to discern that conditions are appropriate toinitiate a closing of the antisurge valve before said turbocompressorreaches a minimum operating rotational speed; and (f) a fourth controlsystem function to generate a second signal to actuate the antisurgevalve towards its closed position after the third control systemfunction has discerned conditions are appropriate to initiate theclosing of the antisurge valve.
 10. The apparatus of claim 9additionally comprising: (a) at least one signal generated by ameasurement system and sensed by the control system; (b) a comparator inthe control system to compare a magnitude of the at least one signalwith a minimum threshold value; and (c) an antisurge actuation signalgenerating function within the control system to initiate a closing ofthe antisurge valve when the magnitude of the at least one signalexceeds the minimum threshold value.
 11. The apparatus of claim 9additionally comprising: (a) a timer, used to measure a time elapsedfrom an initiation of the startup of the turbocompressor; (b) acomparator in the control system to compare the elapsed time with athreshold value; and (c) an antisurge actuation signal generatingfunction within the control system to initiate a closing of theantisurge valve when the elapsed time exceeds the threshold value. 12.The apparatus of claim 9 wherein the second control system functionadditionally comprises: (a) a rotational speed set point generatingfunction within the control system; and (b) a feedback control system tocontrol the turbocompressor rotational speed based on saidturbocompressor rotational speed set point.
 13. The apparatus of claim 9additionally comprising: (a) a plurality of signals generated by ameasurement system and sensed by the control system; (b) a comparator inthe control system to compare a magnitude of each of the plurality ofsignals with a respective minimum threshold value; and (c) an antisurgeactuation signal generating function within the control system toinitiate a closing of the antisurge valve when the magnitude of all theplurality of signals exceed the respective minimum threshold values. 14.The apparatus of claim 9 wherein the fourth control system functionadditionally comprises: (a) a signal indicating a turbocompressoroperating point; (b) a comparator function to compare said signal to avalue representing a surge control line; and (c) a closed-loop controlsystem to manipulate the antisurge valve to keep the turbocompressoroperating point from residing nearer a surge region than the surgecontrol line.
 15. An apparatus for minimizing an energy required tostart a turbocompressor, the apparatus comprising: (a) an antisurgevalve; (b) a constant speed turbocompressor driver having a constantoperating rotational speed; (c) a control system; (d) a first controlsystem function to generate a first signal to open the antisurge valvefully; (e) a second control system function to initiate a startup of theturbocompressor to increase a rotational speed of the turbocompressorfrom a zero rotational speed; (f) a third control system function todiscern that conditions are appropriate to initiate a closing of theantisurge valve before said turbocompressor reaches the constantoperating rotational speed; and (g) a fourth control system function togenerate a second signal to actuate the antisurge valve towards itsclosed position after the third control system function has discernedconditions are appropriate to initiate the closing of the antisurgevalve.